Abstract
Nuclear-resonance scattering of bremsstrahlung, employing nuclear self-absorption from the 3089-keV and the 3684-keV levels in ${}^{13}\mathrm{C},$ has been used to determine the effective temperatures ${T}_{\ensuremath{\parallel}}$ and ${T}_{\ensuremath{\perp}}$ of highly oriented pyrolytic graphite (HOPG) at 10, 295, 495, and 683 K in directions parallel and perpendicular to the hexagonal layers. The effective temperature of isotopic diamond ${(}^{13}\mathrm{C})$ at 295 K was also measured. The bremsstrahlung source was generated using the 4.1-MeV electron accelerator of the University of Stuttgart. In HOPG, the nuclear self-absorption ratios for the parallel and perpendicular directions were quite different, revealing a significant anisotropy in the values of ${T}_{\ensuremath{\perp}}$ and ${T}_{\ensuremath{\parallel}}.$ Comparison between the observed values and those deduced from the vibrational density of states of HOPG agree, within error in ${T}_{\ensuremath{\perp}},$ but the measured ${T}_{\ensuremath{\parallel}}$ is higher by about 20%. In diamond, the measured effective temperature is higher by 32% than the calculated value. The results are discussed in view of the calculated phonon spectra of HOPG and of diamond.
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